1. Field of the Invention
This invention relates to the field of ultra-wideband communication systems. More particularly, it relates to the controlled transmission of ultra-wideband electromagnetic pulses.
2. Background of Related Art
Ultra-wideband (UWB) systems, both for radar and communications applications, have historically utilized impulse, or shock-excited, transmitter techniques in which an ultra-short duration pulse (typically tens of picoseconds to a few nanoseconds in duration) is directly applied to an antenna which then radiates its characteristic impulse response. For this reason, UWB systems have often been referred to as xe2x80x9cimpulsexe2x80x9d radar or communications. In addition, since the excitation pulse is not a modulated or filtered waveform, such systems have also been termed xe2x80x9ccarrier-freexe2x80x9d in that no apparent carrier frequency is evident from the resulting RF spectrum.
To be useful for data communications, previous UWB impulse or carrier-free transmission systems have been limited to ON-OFF keying (binary amplitude shift keying ASK) or pulse position modulation (PPM) since amplitude and/or phase control of the waveform was extremely difficult or impossible to implement. In addition, these previous systems have been fixed bandwidth and fixed frequency with no capability for frequency hopping or dynamic bandwidth control.
Output power and pulse repetition frequency (PRF) of UWB impulse transmitters have also been limited due to fundamental physical limitations of the devices used to generate the ultra-short duration pulses. In particular, high output power and high PRF were mutually exclusive properties of such systems. High output power impulse excitation sources such as bulk avalanche semiconductors, high voltage breakover devices, high voltage Gallium Arsenide (GaAs) thyristors, plasma diodes, stacked arrays of step recovery diodes (SRDs), etc. required hundreds to many thousands of volts for proper operation and, consequently, were limited to PRFs below a few tens of kilohertz due to increased device heating and thermal breakdown at higher PRFs. Lower power devices, such as avalanche transistors, low voltage SRDs, Zener diodes, etc., can operate at PRFs of several megahertz, but produced output powers many orders of magnitude lower. In addition, while the individual devices were typically low cost, they often needed to be hand-selected in order to guarantee avalanche or breakdown characteristics at a particular operating voltage level.
As an example, early versions of UWB impulse transmitters typically generated less than one watt peak microwave output power at a maximum PRF of approximately 10 kHz using baseband impulse excitation powers of tens to several thousand watts. Several laboratory models using these high voltage sources were constructed for radar applications which included ship docking, pre-collision sensing for automobiles, liquid level sensing, and intrusion detection. Although these techniques proved to be reliable, the power efficiency, PRF limitations, size and complicated antenna assemblies limited performance and reproducibility.
Another significant limitation of such impulse-based UWB sources is the fact that the power level decreases with increasing frequency at a rate of approximately 12 dB per octave. This is due to the double exponential nature of the impulse excitation. The output response from a typical impulse source has the form:       p    ⁡          (      t      )        =            t      α        ⁢          ⅇ              (                  1          -                      xe2x80x83                    ⁢                      t            α                          )              ⁢                  u                  -          1                    ⁡              (        t        )            
where xcex1 is the pulse rise time and uxe2x88x921(t) is the unit step function. FIG. 10 shows the output response p(t) versus time. This waveform closely approximates the output seen from the vast majority of impulse sources.
One can now compute the instantaneous pulse power versus frequency (magnitude-squared Fourier transform) as:       P    ⁡          (      f      )        =                    ⅇ        2                    16        ⁢                  xe2x80x83                ⁢                  π          4                      ⁢          xe2x80x83        ⁢          1                        α          2                ⁢                  f          4                    
Note that if the rise time is doubled, the power at any given frequency decreases by 6 dB. Similarly, for a constant peak voltage source, doubling the frequency of operation decreases the output power by 12 dB.
As an example, a 2.5 kW peak power output thyristor-based impulse generator develops only about 1 watt peak power at L-Band (1.5 GHz range) since the vast majority of the impulse energy is produced at significantly lower frequencies. This unused energy is dissipated as heat, subjecting operating circuits to overheating and damage, and limiting the PRF or data rate at which the source can operate reliably. The upper trace in FIG. 11 shows the rapid drop in available power versus frequency from a conventional thyristor-based impulse source.
Another limitation in the use of such techniques is the lack of accurate control of radiated emissions to meet regulatory requirements. Since a short pulse excitation will stimulate the impulse response of an antenna, and a typical wideband antenna has a frequency response extending over many octaves in frequency (an octave of frequency being a doubling of frequency), the radiated spectrum will be extremely broadband, covering hundreds of megahertz (MHz) to several gigahertz (GHz) or more of instantaneous bandwidth. This broad spectrum may overlap many frequencies of operation licensed otherwise by the U.S. Federal Communications Commission (FCC) in the U.S. or by other means in foreign countries, thus presenting a concern to operators or users of allocated frequencies, albeit at very low average power levels.
Thus, conventional UWB signal generation techniques suffer from several shortcomings:
(i) high power operation can only be achieved at reduced PRFs because of device heating;
(ii) practical operational frequencies are limited to well below 5 GHz due to the 12 dB per octave falloff of output impulse energy with increased frequency;
(iii) impulse excitation of an antenna results in a xe2x80x9ccarrier-freexe2x80x9d signal which would uncontrollably overlap frequencies restricted from such use, albeit with low energy densities; and
(iv) modulation techniques are limited to on-off keying and pulse position modulation, with no capability for frequency hopping or for dynamic bandwidth control.
There is a need to achieve a higher output power for long distance communications and for small target detection in the case of a radar system, to develop high PRFs for the transmission of wideband video and data, to produce UWB transmissions at well-controlled center frequencies and bandwidths extending to higher operating frequencies (e.g., millimeter wave), and to allow for newer and more efficient modulation techniques.
The present invention provides a breakthrough in UWB communications in that it generates a waveform adaptive or carrier-controlled UWB signal having a controlled center frequency and an adjustable bandwidth. These adjustments can be performed on a pulse-by-pulse basis, allowing for UWB frequency hopping and adaptive bandwidth control.
One preferred embodiment distinctively utilizes a low-level impulse-gated oscillator to produce an extremely wide bandwidth pulse which can operate at extremely high pulse repetition frequencies (FIG. 1). Precise control of radiated frequency is governed by the choice of oscillator which has a known stable frequency. The oscillator can be fixed frequency or a voltage controlled oscillator (VCO), the latter in particular for UWB frequency hopping applications. Oscillator phase may also be controlled to generate an additional phase modulation. With suitable choice of oscillator and mixer, UWB signals can be generated with center frequencies from near DC to millimeter wave frequencies. Signal bandwidth is governed by a bandpass or pulse shaping filter which, when used to drive a wideband mixer, controls the spectral characteristics of the output waveform. An output bandpass filter further limits out of band energy; and a gated power amplifier is used to amplify the UWB signal to the desired peak power output level.
Another embodiment distinctively utilizes a low-level impulse generator and bandpass or pulse shaping filter without need for a separate oscillator and mixer (FIG. 2). This embodiment is particularly useful for non-agile operation at frequencies below 5 GHz for which sufficient impulse energy can be generated to drive an additional gated power amplifier. This approach is mathematically equivalent to that of FIG. 1 when the oscillator frequency is chosen to be precisely zero. In this case, UWB signal center frequency and bandwidth are directly determined by the characteristics of the bandpass or pulse shaping filter.
Another variant of the impulse-gated oscillator is derived through the use of analog or digital time-gating (FIG. 3). In this embodiment, the low-level impulse excitation is approximated by the response of a set of high-speed switches (FIG. 4). These switches gate the oscillator output ON for a very short time period (FIG. 5). Such time-gating may also be achieved through the use of analog or digital pulse shaping circuitry (FIG. 6).
VCOs in the L-band (1.5 GHz) and in the X-band (10 GHz) region were implemented in exemplary embodiments of the present invention, although it is equally possible that oscillators of other types and of other frequencies can be used. Unlike conventional impulse or xe2x80x9ccarrier-freexe2x80x9d techniques, one aspect of the present invention provides an UWB signal having a well-defined, controllable carrier frequency and bandwidth with the additional capability for independent phase and amplitude modulation.
Each of these UWB transmitters can operate at extremely high data rates, enabling the transmission of high speed data such as real-time digitized video, multiple simultaneous digital voice channels, or other information, as well as the transmission of high PRF pulse trains for radar or ranging applications.
To achieve high power output from any of these UWB sources, a gated power amplifier is used (FIG. 8). The gated power amplifier has the unique feature of high power efficiency as the power amplifier is only turned on for approximately the duration of the UWB pulse.
It is an object of the present invention to provide a frequency and bandwidth adaptive UWB transmitter.
It is a further object to provide an UWB transmitter having controllable spectral features superior to those provided by conventional impulse and xe2x80x9cshock-excitedxe2x80x9d UWB signal generating transmitters.
It is a further object to provide an UWB transmitter system that obviates pulse repetition frequency (PRF) limitations of conventional systems, thus allowing extremely high data rates on the order of hundreds of megabits per second.
It is yet another object of the present invention to provide an UWB transmitter having frequency extensibility to millimeter wave frequencies by suitable selection of an RF carrier and impulse- or time-gating characteristics.
It is an additional object to provide an UWB transmitter having digital amplitude and/or phase control to permit the generation of M-ary communications waveforms such as ultra-wideband quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), etc.
It is a further object to provide an UWB transmitter having frequency agility (e.g., frequency hopping) through direct digital control (DDC) of the RF oscillator center frequency.
It is another object to provide an UWB transmitter having pulse width agility, and thus bandwidth agility, through the use of direct digital control of the time-gating circuitry parameters.
It is also an object to provide an UWB transmitter having 50 xcexa9 impedance matching for ease of fabrication into stripline or microstrip hybrid or multi-chip module (MCM) circuits.
It is another object to provide an UWB transmitter permitting the use of wideband Monolithic Microwave Integrated Circuit (MMIC) power amplifiers to deliver an efficient, significant and accurately controllable amount of transmitter power to an antenna.
It is yet another object to provide an UWB transmitter with a gated power amplifier that achieves high power efficiency because it draws minimal current except during a brief period of time in which the UWB pulse is being generated. Because of the extremely low duty cycle of an UWB waveform, even at high PRFs, the gated power amplifier is important to the UWB transmitter in designs requiring low power consumption such as battery-operated handheld radios, unattended sensors, etc.
These and other objects of the invention will become more readily apparent upon review of the following description. The invention, though, is pointed out with particularity by the appended claims.